The processes of consumption and decomposition are considered ecologically as system-level catabolism. The primary agents of decomposition are bacteria and fungi, often referred to as "microbial biomass."

Microbial production and turnover is measured in a number of indirect ways, both chemical and physiological, as well as in direct fashion, using high-magnification microscopy, or via energetics approaches such as calorimetry.

The microbial biomass, although relatively small (about 200400 g • m-2 within 15 cm of the surface) relative to the total soil organic matter pool, has a rapid turnover time and serves as a principal food source for microbivorous fauna. It is also the source of labile nutrients, available for plant roots and other microbes. Hence the microbial community is indeed the "eye of the needle" through which virtually all of the decomposition carbon and nutrients must pass. In the course of microbial growth and turnover, there is a dynamic process of buildup of macroaggregates, which age and decay into more extensively protected microsites within microaggregates. Roles of fauna in the process of aggregate formation and alteration are also discussed further in Chapter 4.

Some very large strides forward in soil ecology have been taken by investigators who are linking microbial community structure and function. We discuss several examples in Chapter 8, regarding the effects of invasive plant species on microbial communities. It is apparent that a combined or synthetic approach using aspects of functional measures such as enzyme activities, when combined with the qualitative measures such as PLFA and the more quantitative measures of using 16S rRNAgene probes and cDNA analyses, along with the immunofluores-cent approaches presented by Artursson and Jansson (2003), will yield considerable dividends in future studies.

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